Many silicon wafers are used in semiconductor industries, and growth of a silicon single crystal being a base material thereof is an important technology. To grow the silicon single crystal, a floating zone (FZ) method for locally melting a silicon ingot to form a single crystal by heating using an induction coil and a Czochralski (CZ) method for melting a silicon raw material in a crucible by heating using a heater and pulling a single crystal from a molten melt are available. The crucible in the CZ method generally has a double structure constituted of a quartz crucible made of silicon and oxygen and a graphite crucible that supports the quartz crucible to prevent a shape of the quartz crucible from collapsing due to softening at high temperatures. According to the CZ method, oxygen eluted from the quartz crucible is taken into silicon in a grown crystal, and oxygen precipitates are formed in a wafer sliced out from this crystal by, e.g., a heat treatment in a device and exercise a gettering effect of capturing impurities during a device process. Further, since increasing a diameter is relatively easier in the CZ method than in the FZ method, the CZ method has become mainstream as a method for industrially growing a silicon single crystal.
Since movement of electrons or holes enables a device formed on a silicon wafer to operate, a dislocation in the wafer might cause a problem such as leak of an electric current. Therefore, the silicon wafer serving as a raw material in formation of a device must have no dislocation. Thus, an original crystal from which silicon wafers are to be sliced out must be a single crystal having no dislocation. Since the crystal is grown at high temperatures, when a dislocation occurs during crystal growth, the dislocation glides or increases, resulting in occurrence of many dislocations. Since a wafer sliced out from such a crystal having dislocations includes many dislocations, an advanced device cannot be fabricated thereon. Thus, the occurrence of dislocations is a serious problem in the crystal growth. However, in spite of studies conducted over the years, the occurrence of dislocations is yet to be completely prevented.
Causes of the occurrence of dislocations during the crystal growth in the CZ method are considered to include internal stress during the crystal growth, various kinds of sparingly-soluble materials, and others. As regards the internal stress that contributes to the occurrence of dislocations in a crystal, for example, greatly raising a growth rate of the crystal increases solidification latent heat generated when a liquid changes into a solid, and a crystal growth interface that is an isotherm of a melting point has an upwardly protruding shape to cause a rise in its height. When the height of the crystal growth interface rises, a temperature gradient in a direction vertical to a crystal growth axis increases, enlarging stress in a central portion of the crystal. It is empirically known that the increase in this stress above a certain level leads to the occurrence of dislocations. To avoid this occurrence, for example, removing the generated solidification latent heat by enhancing cooling of the crystal lowers the height of the crystal growth interface, and the stress can be consequently decreased to prevent dislocations from occurring. An easier method is to decrease the solidification latent heat by reducing a growth rate. In general, a crystal is ordinarily grown within the limit of growth rates that avoid the occurrence of dislocations due to internal stress, and the occurrence of dislocations due to the internal stress is not a particularly serious problem.
Sparingly-soluble materials as another cause of the occurrence of dislocations are considered to include an impurity derived from in-furnace components, e.g., a graphite material, a heat-insulating member, or a wire present in the furnace, SiO2 provided when the quartz crucible is partially delaminated due to crystallization/degradation/air bubble opening of the quartz crucible, Sb provided when volatile silicon oxide (SiO) resulting from a reaction of oxygen eluted into a silicon melt from the quartz crucible and silicon adheres to and solidifies on a cooled portion such as a tip of a straight body of the crucible or a chamber and it again falls into the raw material melt, and solidification of the raw material melt caused due to unevenness or fluctuation of temperatures of the raw material melt. For example, contriving shapes of the in-furnace components can relatively easily develop a solution for the impurity derived from the components among others.
As to generation of the SiO2 sparingly-soluble materials due to degradation of the quartz crucible, for example, in Patent Literature 1, degradation prevention is taken by adjusting a furnace internal pressure during an operation. Furthermore, various technologies that improve quality of the quartz crucible itself are disclosed.
As regards the problem that the volatile SiO solidifies and falls, for example, Patent Literature 2 discloses a technology that straightens the volatile SiO or a gas such as CO or CO2 produced in a heater section with the use of, e.g., an argon gas flowing from above by providing a cylinder (a gas flow guide cylinder) that surrounds a crystal and a collar at a lower end thereof, avoiding adhesion to components above a crucible. Moreover, Patent Literature 3 discloses that an outer peripheral portion of a collar is extended to an upper portion of a straight body of a crucible to keep an upper end portion of the straight body of the crucible warm, preventing SiO from adhering. Additionally, although an object is different, Patent Literature 4 discloses a structure that is a combination of an inverse conical heat shielding member having a heat insulator and an outer heat-insulating member of a straight body of a crucible, each of Patent Literatures 5 and 6 discloses a heat-insulating member that projects to a position near a straight body of a crucible above a heater, and Patent Literature 7 discloses a radiant heat shield that projects to an upper portion of a sidewall and an inner side of a crucible. These patent literatures are considered to have an effect to keep the straight body of the crucible warm and prevent SiO from adhering.
Finally, as to the problem of solidification of the raw material melt, for example, Patent Literature 8 discloses a technology of reflecting radiant heat by a heat shielding ring to keep the vicinity of an interface warm. Further, Patent Literature 9 discloses a technology of keeping a space below an upper ring warm by the upper ring installed above a quartz crucible to enable suppressing solidification of a raw material melt.
As described above, various measures have been taken to several conceivable causes of the occurrence of dislocations thus far as attempts of prevention. However, taking these measures fails to completely suppress the occurrence of dislocations in a crystal, and efforts to reduce the occurrence of dislocations, e.g., modification of the quartz crucible or optimization of various operation conditions are continued on a daily basis. Further, in recent years, growth of demands for crystals that have no defect when the crystals are grown has become prominent. To grow the defect-free crystal, a temperature gradient within a crystal plane must be maintained uniform. To uniform this temperature gradient within the crystal plane, assuring a longer distance between the gas flow guide cylinder or the heat shielding ring and a liquid surface of the raw material melt has been carried out, heat retaining properties of the liquid surface of the raw material melt are consequently reduced, and the raw material melt is solidified, resulting in a cause of the occurrence of dislocations.